Three-dimensional images are acquired by various imaging modalities including computed tomography (CT) and magnetic resonance imaging (MRI). Computed tomography traditionally serves as the modality used in creating treatment plans for radiation therapy. A CT scan accurately represents the entire geometry of a subject, and the CT values can be directly converted to electron densities, which are used in calculating the radiation dose distribution within a subject. However, CT images have poor soft tissue contrast, and CT scanning exposes the subject to additional radiation dose. Compared to CT, the MRI modality has superior soft-tissue contrast, is free of ionizing radiation, and could provide functional information within the subject such as metabolism.
Currently, MRI images are mainly used to complement CT images for more accurate anatomical structure contouring and tumor targeting, therefore an image of the MRI scan of target subject needs to be aligned to the corresponding image of the CT scan. The alignment of MRI and CT images are not exact due to the acquisition of MRI and CT images on different scanning machines, thus contributing to inaccuracy in tumor targeting. In MRI-guided radiation therapy where only MRI scans are used, there is no need for the alignment to CT images and thus tumor target delineation is more accurate. However, MRI intensity values are not directly related to electron densities used for dose calculations, therefore there is an interest in a method for accurately converting MRI images into corresponding images with electron density values, usually CT (referred to as pseudo-CT or pCT) images.
The journal article Edmund and Nyholm, “A Review of Substitute CT Generation for MRI-only Radiation Therapy”, Radiat Oncol 12:28 (2017), doi:10.1186/s13014-016-0747-y, reviews various methods employed to create a pCT, including the atlas method and the voxel method. As described in the journal article Dowling et al., “An Atlas-based Electron Density Mapping Method for Magnetic Resonance Imaging (MRI)-Alone Treatment Planning and Adaptive MRI-Based Prostate Radiation Therapy”, Int J Radiat Oncol Biol Phys 83, 5 (2012), doi:10.1016/j.ijrobp.2011.11.056, an atlas image is a pre-existing image used as reference to assist in generating a derived image. In the context of generating a pCT, an atlas MRI image and an atlas CT image are used as references for generating a derived CT image from a new MRI image of target subject. The atlas MRI is aligned to the target MRI and the same transformation is applied to the collection of atlas CTs that are fused into a single pCT image. However, there will be certain errors in the registration or alignment of the atlases or from the atlas image to the target MRI image. A voxel method mainly uses the intensity values of voxels from MRI images and CT images to implement a conversion method to create pseudo-CT images without the alignment between MRI images from target subject and training subject.